The palmaris longus is a long, slender muscle in the forearm that is classified as vestigial, meaning it has lost most of its original function during human evolution. It is situated superficially on the palm-side of the forearm, running from the elbow down to the wrist. Determining whether you possess this anatomical feature is straightforward and can be done by a simple visual examination of the wrist. Its presence is highly variable across the human population.
The Simple Self-Test
To check for the presence of the palmaris longus, extend one arm out in front of you with the palm facing upward. Bring your thumb and little finger together, touching their tips. While maintaining this hand position, gently flex your wrist upward toward the ceiling.
This combination of movements, known as the Schaeffer’s test, causes the forearm tendons to become taut and visible at the wrist crease. If the muscle is present, a raised, cord-like tendon will become visible in the center of the wrist. It will appear as the most prominent line between the two other large tendons on either side.
If you do not see a distinct, raised line in the middle of the wrist, the palmaris longus is likely absent in that arm. Since the absence can be unilateral, repeat the test on your other arm to determine its presence there as well.
What the Palmaris Longus Muscle Does
The anatomical role of the palmaris longus is relatively minor in modern humans. When present, its primary action is to assist in the weak flexion of the wrist joint, helping to bend the hand forward toward the forearm.
A secondary function is to apply tension to the palmar aponeurosis, the thick layer of fascia in the palm of the hand. This action contributes to stabilizing the skin in the palm during forceful gripping or grasping movements. Compared to the larger flexor muscles in the forearm, the power contribution of the palmaris longus is minimal.
Why Not Having It Is Normal
The absence of the palmaris longus is common and causes no functional deficit or loss of strength in the hand or wrist. The rate of absence varies widely across the global population, ranging from 1.5% to over 37%. A traditionally accepted average for its absence falls around 10% to 15% of people worldwide.
The function the palmaris longus provides is easily compensated for by the stronger, surrounding forearm muscles. The flexor carpi radialis and the flexor carpi ulnaris muscles are capable of handling the work of wrist flexion. Because it is non-essential, its absence is simply a benign anatomical variation.
This dispensability gives the palmaris longus tendon a prominent place in modern reconstructive surgery. When a tendon graft is needed to repair a damaged ligament or tendon elsewhere in the body, the palmaris longus is frequently the tendon of choice. Surgeons can harvest this tendon without any resulting impairment to the patient’s hand or wrist function.